Fix #77978: Dirname ending in colon unzips to wrong dir

When making the relative path, we must not stop on a `:\` sequence in
the middle of the filename. This is only significant on Win

Fix #77978: Dirname ending in colon unzips to wrong dir

When making the relative path, we must not stop on a `:\` sequence in
the middle of the filename. This is only significant on Windows as it
may indicate an absolute filename, but this is already checked at the
beginning of the function.

Note that the bug and this patch affects all systems. However, on
Windows the file is no longer extracted at all, since Windows NTSF does
not allow filenames containing colons.

Closes GH-7528.

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